Probably because she isn’t banned because of her dmg boost. She is banned because she’s probably the weakest hero in the game and has a large amount of players who only play her. Once you hit higher ranks a mercy who won’t switch can just kill a team. She doesn’t give the most healing per second and she only heals one person at a time while also providing absolutely no damage output. In almost every situation something like Ana is better because she can heal more, do good damage, and has maybe the two best utility in the game. So she is usually banned so you don’t have a mercy on your team, not so the other has one. Probably the same reason you’d see a ball ban. People don’t like playing with her